This invention relates to improvements in bistatic passive radar systems. In ordinary (monostatic) radar systems, the radar transmitter and radar receiver are located at the same site. The transmitter emits pulses of electromagnetic energy which travel through space to electrically conductive targets and are reflected back from the targets to the receiver. The range of the targets is determined by the time it takes the pulse of electromagnetic energy to travel from the transmitter to the target and then back to the receiver. The bearing of the target is determined by focusing the transmitted pulse in a narrow beam and relatively slowly scanning the beam over the area to be searched. Thus, the bearing of the target is the same as the bearing of the antenna at the time the reflected pulse is received.
Monostatic radar has a serious disadvantage when used on military aircraft and ships in that the presence and bearing of the aircraft or ship can be detected at relatively long range (hundreds of miles) by detecting the electromagnetic pulses emitted by the radar transmitter. To overcome this disadvantage, bistatic passive radar systems were developed.
Typical prior art bistatic passive radar systems have two receivers but no transmitter. Instead of using a transmitter to illuminate the target, bistatic passive radar systems utilize the radar illumination provided by any monostatic radar transmitter within the search range of the bistatic passive radar system. One receiver of the bistatic passive radar system locks onto the host transmitter's pulse train, measures the angle between its own azimuth reference and the line of sight to the host transmitter, and determines the host transmitter's scan rate. From these data, a plan position indicator (PPI) display is generated. The other receiver of the bistatic passive radar system picks up target reflections from the host transmitter and displays them on the PPI display. Thus a display of targets is generated without emission of any appreciable levels of electromagnetic energy from the bistatic passive radar. Therefore, the bistatic passive radar system cannot be detected from a distance by its radiation.
Targets appear at their proper bearing from the transmitter on the PPI display of a bistatic passive radar system, but the accuracy of their range is dependent on accurate measurement of the distance D between the host transmitter and the bistatic passive radar system. In the past, however, there has not been any means for actually measuring the distance D between the host transmitter and the bistatic passive radar system except when the position of both were known. Therefore, the distance D between the host transmitter and the bistatic passive radar system usually was estimated. This estimation may be accurate in cases where landmarks are present on the PPI display, and map overlays can be used to estimate the position of the host transmitter, or in cases where the position of the host transmitter is known. But in deep ocean situations, where there are no landmarks, and where the host transmitter may be an enemy surface vessel, the estimated range of the host transmitter may be grossly inaccurate and therefore the range of the targets on the PPI display of the bistatic passive radar system would also be grossly inaccurate, although their bearings in relation to the position of the host transmitter would be correctly displayed.
The principal object of this invention is to provide a system for accurately determining the range of selected targets from the host transmitter without landmarks or other clues as to the position of the host transmitter so that the bistatic passive radar system can be successfully employed in deep ocean situations or in similar situations where no landmarks exist to locate the position at the host transmitter. When multiple targets at different ranges and bearings exist, the presention invention provides increased accuracy through integration of solutions for the same distance D.